1. Field of the Invention
The present invention relates to an image formation method using an electrophotographic photoconductor and a two-component developer in combination.
2. Discussion of Background
According to the electrophotographic process, a latent electrostatic image is formed on the surface of a photoconductor through the steps of charging and light exposure, and the formed latent electrostatic image is developed with a developer to obtain a toner image. The toner image thus formed is transferred to an image receiving material and fixed thereon, whereby a visible toner image can be obtained on the image receiving material. To develop the latent electrostatic image, there are conventionally employed a powder cloud development method, a cascade development method, a magnetic-brush development method, and so on. In particular, the magnetic-brush method is widely employed.
A two-component dry developer for use with the magnetic-brush development method is composed of a magnetic carrier component, for example, comprising ferrite particles, and a toner component, for example, comprising toner particles containing a coloring agent and a resin. The carrier particles and toner particles are triboelectrically charged and retained under such conditions. When the two-component developer comes in close vicinity to the latent electrostatic image formed on the photoconductor, the toner particles for use in the two-component developer are separate from the carrier and drawn toward the latent electrostatic image if the force of an electric field for constituting the latent electrostatic image overcomes the triboelectric attraction of the toner particles for the carrier particles. In this case, the toner particles are attracted and attached to the latent electrostatic image, whereby the latent electrostatic image on the photoconductor is made visible. The toner component for use in the developer is thus consumed in the course of development, and the two-component developer is repeatedly and continuously used with the toner component being replenished in the developer.
Such an electrophotographic process is carried out in the conventional copying machines. In addition to the copying machines, laser beam printers adopting the electrophotographic process have been currently on the market to output the data with the recent spread of computers. In line with such a tendency, the electrophotographic image forming apparatus is required to produce high quality images. From the viewpoint of the employed developer, high quality images can be obtained by decreasing the particle diameters of both the toner particles and the carrier particles. In particular, a decrease in particle diameter of the toner particles is considered to be effective in faithfully reproducing a fine latent image on the photoconductor.
However, the smaller the particle diameter of the toner particles, the more the triboelectric charging characteristics of the toner particles themselves. Further, the adhesion between the toner particles determined by the van der Waals force is increased. As a result, there is a possibility that a toner image portion with a large toner deposition amount formed on the photoconductor cannot be completely transferred to a transfer sheet such as a sheet of paper. In other words, non-transferred spots may appear in the form of worm-eaten spots.
To solve the above-mentioned problem, some proposals are made with special attention being paid to the surface properties of the photoconductor on which toner images are to be formed. For example, Japanese Laid-Open Patent Application 5-188643 discloses a toner that is produced by polymerization so as to be composed of toner particles classified in a narrow particle size distribution. However, the particle size of the toner particles obtained by the method disclosed in this application is still insufficient when the carrier with a smaller particle diameter is used in combination. Further, by the above-mentioned preparation method, the toner needs a coating film in which inorganic particles are dispersed, so that there is some difficulty in preparing the toner particles.
In light of the above-mentioned prior art, there is an increasing demand for establishment of an electrophotographic image formation method capable of more conveniently producing high quality images.